[PATCH v1 1/2] KVM: arm64: Fix page leak in user_mem_abort() on atomic fault

[Fuad Tabba <tabba@google.com>]

When a guest performs an atomic/exclusive operation on memory lacking
the required attributes, user_mem_abort() injects a data abort and
returns early. However, it fails to release the reference to the
host page acquired via __kvm_faultin_pfn().

A malicious guest could repeatedly trigger this fault, leaking host
page references and eventually causing host memory exhaustion (OOM).

Fix this by consolidating the early error returns to a new out_put_page
label that correctly calls kvm_release_page_unused().

Fixes: 2937aeec9dc5 ("KVM: arm64: Handle DABT caused by LS64* instructions on unsupported memory")
Signed-off-by: Fuad Tabba <tabba@google.com>
---
 arch/arm64/kvm/mmu.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index ec2eee857208..e1d6a4f591a9 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1837,10 +1837,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	if (exec_fault && s2_force_noncacheable)
 		ret = -ENOEXEC;
 
-	if (ret) {
-		kvm_release_page_unused(page);
-		return ret;
-	}
+	if (ret)
+		goto out_put_page;
 
 	/*
 	 * Guest performs atomic/exclusive operations on memory with unsupported
@@ -1850,7 +1848,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	 */
 	if (esr_fsc_is_excl_atomic_fault(kvm_vcpu_get_esr(vcpu))) {
 		kvm_inject_dabt_excl_atomic(vcpu, kvm_vcpu_get_hfar(vcpu));
-		return 1;
+		ret = 1;
+		goto out_put_page;
 	}
 
 	if (nested)
@@ -1936,6 +1935,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 		mark_page_dirty_in_slot(kvm, memslot, gfn);
 
 	return ret != -EAGAIN ? ret : 0;
+
+out_put_page:
+	kvm_release_page_unused(page);
+	return ret;
 }
 
 /* Resolve the access fault by making the page young again. */
-- 
2.53.0.473.g4a7958ca14-goog